1. Field of the Invention
The present invention relates to a semiconductor laser device and a method of fabricating the same, and more particularly, it relates to a semiconductor laser device having a convex ridge portion and a method of fabricating the same.
2. Description of the Background Art
A semiconductor laser device having a convex ridge portion serving as a current path is known in general. For example, Japanese Patent Laying-Open No. 2002-252421 discloses this type of semiconductor laser device.
FIG. 50 is a sectional view for illustrating a conventional semiconductor laser device having a ridge portion. The structure of the conventional semiconductor laser device having a ridge portion is described with reference to FIG. 50.
In the conventional semiconductor laser device having a ridge portion, an n-type buffer layer 202 of n-type GaInP, an n-type cladding layer 203 of n-type AlGaInP, an emission layer 204 including a multiple quantum well (MQW) active layer of GaInP/AlGaInP and a p-type first cladding layer 205 of p-type AlGaInP are successively formed on an n-type GaAs substrate 201, as shown in FIG. 50.
A mesa (trapezoidal) ridge portion constituted of a p-type second cladding layer 206 of p-type AlGaInP, an intermediate layer 207 of p-type GaInP and a contact layer 208 of p-type GaAs is formed on a prescribed region of the upper surface of the p-type first cladding layer 205. This ridge portion is formed in a striped (elongated) shape.
Current blocking layers 209 prepared by stacking n-type AlInP layers and n-type GaAs layers are formed to cover the upper surface of the p-type first cladding layer 205 and both side surfaces of the ridge portion while exposing only the upper surface of the ridge portion (contact layer 208). A p-type cap layer 210 of p-type GaAs is formed to cover the exposed upper surface of the ridge portion and the upper surfaces of the current blocking layers 209.
A p-side electrode 211 is formed on a portion of the p-type cap layer 210 around the aforementioned ridge portion. An n-side electrode 212 is formed on the back surface of the n-type GaAs substrate 201.
A process of fabricating the conventional semiconductor laser device having a ridge portion is now described with reference to FIG. 50. First, the n-type buffer layer 202 of n-type GaInP, the n-type cladding layer 203 of n-type AlGaInP, the emission layer 204 including the multiple quantum well (MQW) active layer of GaInP/AlGaInP, the p-type first cladding layer 205 of p-type AlGaInP, the p-type second cladding layer 206 of p-type AlGaInP, the intermediate layer 207 of p-type GaInP and the contact layer 208 of p-type GaAs are successively formed on the n-type GaAs substrate 201 by MOVPE (metal organic vapor phase epitaxy).
Then, SiO2 layers (not shown) are formed on the contact layer 208 by photolithography and etching at a prescribed interval. The SiO2 layers are employed as masks for etching the p-type second cladding layer 206, the intermediate layer 207 and the contact layer 208, thereby forming the mesa (trapezoidal) ridge portion consisting of the p-type second cladding layer 206, the intermediate layer 207 and the contact layer 208 in the striped shape on the central portion of the p-type first cladding layer 205.
Then, an SiO2 layer (not shown) formed on the ridge portion is employed as a mask for growing the current blocking layers 209 consisting of the n-type AlInP layers and the n-type GaAs layers to cover the upper surface of the p-type first cladding layer 205 and both side surfaces of the ridge portion. Thereafter the SiO2 layer (not shown) is removed from the ridge portion.
Thereafter the p-type cap layer 210 of p-type GaAs is formed by MOVPE to cover the exposed upper surface of the ridge portion and the upper surfaces of the current blocking layers 209. The p-side electrode 211 is formed on the portion of the p-type cap layer 210 around the aforementioned ridge portion by the lift off method.
The back surface of the n-type GaAs substrate 201 is etched for thereafter forming the n-side electrode 212 on this back surface. The conventional semiconductor laser device having a ridge portion is formed in the aforementioned manner.
FIG. 51 is a sectional view illustrating the conventional semiconductor laser device shown in FIG. 50 in a state mounted on a submount 251 in a junction-down system. According to the junction-down system, the semiconductor laser device is mounted on the submount 251 from the surface closer to the emission layer (active layer) 204. Referring to FIG. 51, a projecting portion of the p-side electrode 211 provided on the surface of the aforementioned conventional semiconductor laser device is directed downward and mounted on a metal film (electrode) 252 of the submount 251 through a welding material 253 consisting of a low melting point metal such as solder. In this case, the submount 251 also has a function of a heat sink absorbing heat of the semiconductor laser device and dissipating the same outward in general. Therefore, the heat generated from the semiconductor laser device is dissipated by the submount 251 from the aforementioned ridge portion through the p-type cap layer 210, the p-side electrode 211, the welding material 253 and the metal film 252.
When the aforementioned conventional semiconductor laser device is mounted on the submount 251 in the junction-down system, however, the heat generated from the semiconductor laser device is dissipated by the submount 251 through the p-type cap layer 210 of p-type GaAs having lower thermal conductivity than the welding material 253 of a low melting point metal such as solder, to disadvantageously reduce heat dissipativity. Therefore, the conventional semiconductor laser device is disadvantageously reduced in reliability (lifetime).
In the aforementioned conventional semiconductor laser device, further, three crystal growth steps must be carried out in total by MOVPE for growing the layers from the n-type buffer layer 202 to the contact layer 208, the current blocking layers 209 and the p-type cap layer 210 respectively. Consequently, the fabrication process for the semiconductor laser device is disadvantageously complicated.
When mounted on the submount 251 in the junction-down system, further, the aforementioned conventional semiconductor laser device is so easily inclined with respect to the submount 251 that the welding material 253 such as solder adheres to the side end surfaces of the inclined semiconductor laser device to easily electrically short the n-type semiconductor layers 202 and 203 and the p-type semiconductor layers 205 to 208 holding the emission layer 204 including the MQW active layer therebetween. Consequently, the fabrication yield is disadvantageously reduced.
When the aforementioned semiconductor laser device is mounted on the submount 251 in the junction-down system, in addition, only the projecting portion of the p-side electrode 211 comes into contact with the metal film 252 of the submount 251, to disadvantageously easily apply stress to the ridge portion located under the projecting portion of the p-side electrode 211. When stress is applied to the ridge portion, the operating current and the operating voltage are disadvantageously increased. When stress is applied to the ridge portion, further, the intensity ratio of polarization between a TE mode having an electric field component in a direction parallel to the emission layer 204 including the MQW active layer and a TM mode having an electric field component in a direction perpendicular to the emission layer 204 (intensity of TE mode/intensity of TM mode: polarization ratio) is disadvantageously reduced in light emitted from the semiconductor laser device.